Aqueous polytetrafluoroethylene (PTFE) dispersions comprising submicron PTFE particles and methods for making such submicron PTFE dispersions are generally known in the art. The submicron PTFE dispersions that result from processes known in the art are typically sold and used in their dispersed form and may be incorporated directly into a desired application system. In addition, such known submicron PTFE dispersions may be used as “master dispersions” or concentrated dispersions that may be diluted or let down to a desired final concentration in the application system of choice.
Typically, these known dispersions of submicron PTFE particles are formed via polymerization in water with the use of dispersing agents and mild agitation at elevated temperatures and pressures. However, many such known submicron PTFE dispersions are characteristically unstable over time and with changing temperature. In addition, these known submicron PTFE dispersions tend to coagulate over time and may be highly sensitive to physical agitation or mechanical handling, whereby they undergo smearing. Thus, a need exists for improved methods of forming submicron PTFE dispersions in aqueous and organic media.
Similarly to PTFE dispersions, dry PTFE powder products are known in the art and are generally available in the industry. Several manufacturers in the fluoropolymer industry produce PTFE powders, and some of these manufacturers describe the PTFE particle size in their powders as being “submicron” or capable of being dispersed to submicron size. Such PTFE powders that are known in the art and that relate to the methods and products of the present invention primarily include “fine powder” PTFE products (which are also known as PTFE coagulated dispersions, PTFE coagulated solids, or PTFE that is formed by emulsion or dispersion polymerization rather than suspension polymerization).
Such fine powder PTFE products, coagulated PTFE solids products, and PTFE products formed by emulsion polymerization typically consist of loose agglomerates of primary PTFE particles, wherein the primary particle size of the PTFE particles is less than 1.00 μm and may range from about 0.1 μm to about 0.5 μm. During solids recovery, the primary PTFE particles may be coagulated through the use of salts, such as ammonium carbonate, in a process known as “salting out,” through pH adjustments, through chemical neutralization with the use of a surfactant, and so forth, and then recovered by various methods such as decantation. Specifically, during emulsion polymerization, the PTFE is polymerized using a wetting agent in order to avoid the formation of large, granular PTFE crystalline particles.
End uses for such commercially available fine powder PTFE products in industrial applications have typically included the formation of PTFE tape, PTFE tubing, and sintered PTFE sheets or tape. Typically, fine powder PTFE products undergo a process commonly termed in the art as “paste extrusion,” whereby the PTFE is placed into a carrier and is extruded to form such tapes, tubing, and sheets. It is believed that the ability of fine powder PTFE products to undergo paste extrusion successfully stems from the elongation ratio of the PTFE particles. As the PTFE industry has evolved, however, irregularities in such tapes and sheets have been observed, leading those skilled in the art to become concerned about reducing the particle size of the PTFE particles in such fine powder PTFE products. Also, such commercially available fine powder PTFE products have been known to exhibit significant stickiness and fibrillation.
Various examples of PTFE powders exist in the art. For example, ICI Fluoropolymers produces a commercially available product known as FLUOROGLIDE FL 1700, which acts as a lubricant and is described in product literature as a white, finely divided, coarse, low molecular weight PTFE powder that is intended for processing via high shear mixing to achieve submicron particle size. Information known in the art about FLUOROGLIDE FL 1700 indicates that the material is used primarily as an additive to improve wear resistance and enhance lubricity, non-stick and frictional characteristics of a host medium.
Additionally, a PTFE fine powder product manufactured by DuPont is known in the art and commercially available as ZONYL Fluoroadditive MP 1100. This product is described as a white, free-flowing PTFE powder designed for use as an additive in other materials, for example, as a process aid, a grease additive, an oil additive, or a thickener for an oil or grease, in order to impart low surface energy and other fluoropolymer attributes to the given system. The DuPont ZONYL MP 1100 product may also be used alone as a dry lubricant.
The desire of fluoropolymer manufacturers to create improved processes for making submicron PTFE powders and submicron PTFE dispersions stems from the wide array of end uses that exists for small particle size or submicron PTFE. For example, small amounts (e.g., about 0.1 to 2% by weight) of powdered PTFE may be incorporated into a variety of compositions to provide the following favorable and beneficial characteristics: (i) in inks, PTFE provides excellent mar and rub resistance characteristics; (ii) in cosmetics, PTFE provides a silky feel; (iii) in sunscreens, PTFE provides increased shielding from UV rays or increased SPF (sun protection factor); (iv) in greases and oils, PTFE provides superior lubrication; and (v) in coatings and thermoplastics, PTFE provides improved abrasion resistance, chemical resistance, weather resistance, water resistance, and film hardness.
Other, more specific end uses for submicron PTFE powders and dispersions include, but are certainly not limited to: (i) incorporating a uniform dispersion of submicron PTFE particles into electroless nickel coatings to improve the friction and wear characteristics of such coatings (Hadley et al., Metal Finishing, 85:51-53 (December 1987)); (ii) incorporating submicron PTFE particles into a surface finish layer for an electrical connector contact, wherein the PTFE particles provide wear resistance to the surface finish layer (U.S. Pat. No. 6,274,254 to Abys et al.); (iii) using submicron PTFE particles in a film-forming binder as a solid lubricant in an interfacial layer, wherein the interfacial layer is part of an optical waveguide fiber (U.S. Pat. No. 5,181,268 to Chien); (iv) using a submicron PTFE powder (along with a granulated PTFE powder and TiO2) in a dry engine oil additive, wherein the additive increases the slip characteristics of the load bearing surfaces (U.S. Pat. No. 4,888,122 to McCready); and (v) combining submicron PTFE particles with autocatalytically-applied nickel/phosphorus for use in a surface treatment system for metals and metal alloys, wherein the PTFE imparts lubrication, low friction, and wear resistance to the resulting surface (“Niflor Engineered Composite Coatings,” Hay N., International, Ltd. (1989)). Additional specific examples of end uses for PTFE involve incorporating PTFE into engine oils, using PTFE as a thickener in greases, and using PTFE as an industrial lubricant additive. Willson, Industrial Lubrication and Tribology, 44:3-5 (March/April 1992).
Furthermore, the use of PTFE powders as additives to the polymers used to make certain fibers is important in that the PTFE powder improves the non-wetting properties of the fibers and the textiles made from such fibers. Thus, fibers incorporating PTFE powder additives are useful in industrial textiles such as textile articles used for filtration and dewatering processes. Such fibers incorporating PTFE powder additives may also be used in producing carpets, fabrics for sportswear and outerwear, hot-air balloons, car and plane seats, umbrellas, and the like. The incorporation of PTFE into such textiles results in many advantages, such as the textile articles being easier to clean. Furthermore, the incorporation of PTFE into certain fibers may provide those fibers with improved tensile strength.
It is important to note, however, that those skilled in the art related to PTFE powders and PTFE dispersions have experienced difficulties in knowing how to obtain colloidal PTFE that is properly stabilized and dispersed in media such as mineral oils, other synthetic oils, resins, polymers, and so forth. It has been realized in the industry that a truly stable colloidal suspension of PTFE particles requires the particles to be of submicron size and to have suitable surface chemistry. Thus, novel methods of forming stable colloidal suspensions or dispersions of submicron PTFE particles are important and desired in the industry.
For many applications or end uses incorporating submicron PTFE powders and submicron PTFE dispersions (such as the end uses described above), the beneficial effects being imparted to the application or end use system are derived from the chemical inertness of the PTFE particles and/or the low coefficient of friction of the PTFE particles. In addition, because submicron PTFE particles have such low particle size, they possess a significantly higher ratio of surface area to weight when compared to larger PTFE particles. Thus, submicron PTFE particles (as compared to larger PTFE particles) are better able to supply their useful effects to a desired application system when incorporated at the same weight load. Therefore, novel methods for preparing submicron PTFE powders and submicron PTFE dispersions would be advantageous to many end uses, products, and/or compositions.
In short, a need exists for simple and straightforward methods for preparing submicron PTFE powder that is free-flowing, that is readily dispersible in various application systems, and that tends not to self-agglomerate (so that neither costly chemical additives nor a substantial amount of energy is required to disperse the submicron PTFE powder into a desired application system). In addition, a need exists for improved methods of forming both aqueous and organic dispersions of submicron PTFE particles, wherein such dispersions are more stable, have less tendency to coagulate over time, and are less physically sensitive to temperature changes and physical agitation. The disclosure of the present invention addresses these and other needs.